1. Field of the Invention
The field of this invention relates to a process for reducing the sulfur content of coal.
2. Prior Art
The problem of air pollution due to the emission of sulfur oxides when sulfur-containing fuels are burned has received increasing attention in recent years. It is now widely recognized that sulfur oxides can be particularly harmful pollutants since they can combine with moisture to form corrosive acidic compositions which can be harmful and/or toxic to living organisms in very low concentrations.
Coal is an important fuel, and large amounts are burned in thermal generating plants primarily for conversion into electrical energy. One of the principal drawbacks in the use of coal as a fuel is that many coals contain amounts of sulfur which generate unacceptable amounts of sulfur oxides on burning. For example, coal combustion is by far the largest single source of sulfur dioxide pollution in the United States at present, and currently accounts for 60 to 65% of the total sulfur oxide emissions.
The sulfur content of coal, nearly all of which is emitted as sulfur oxides during combustion, is present in essentially two forms: inorganic, primarily metal pyrites, and organic sulfur. The inorganic sulfur compounds are mainly iron pyrites, with lesser amounts of other metal pyrites and metal sulfates. The organic sulfur may be in the form of thiols, disulfide, sulfides, and thiophenes chemically associated with the coal structure itself. Depending on the particular coal, the sulfur content can be primarily in the form of either inorganic sulfur or organic sulfur. Distribution between the two forms varies widely among various coals. For example, both Appalachian and Eastern interior coals are known to be rich in pyritic and organic sulfur. Generally, the pyritic sulfur represents from about 25% to 70% of the total sulfur content in these coals.
Heretofore, it was recognized that it would be highly desirable to remove (or at least lower) the sulfur content of coal prior to combustion. In this regard, a number of processes have been suggested for reducing the inorganic (pyritic) portion of the sulfur in coal.
For example, it is known that at least some pyritic sulfur can be physically removed from coal by grinding the coal, and subjecting the ground coal to froth flotation or washing processes. While such processes can desirably remove some pyritic sulfur and ash from the coal, these processes are not fully satisfactory because a significant portion of the pyritic sulfur is not removed. Attempts to increase the portion of pyritic sulfur removed have not been successful because these processes are not sufficiently selective. Because the process is not sufficiently selective, attempts to increase pyrite removal can result in a large portion of coal being discarded along with ash and pyrite. Organic sulfur cannot be physically removed from coal.
There have also been suggestions heretofore to chemically remove pyritic sulfur from coal. For example, U.S. Pat. No. 3,768,988 to Meyers, issued Oct. 30, 1973, discloses a process for reducing the pyritic sulfur content of coal involving exposing coal particles to a solution of ferric chloride. The patent suggests that in this process ferric chloride reacts with pyritic sulfur to provide free sulfur according to the following reaction process: EQU 2FeCl.sub.3 +FeS.sub.2 .fwdarw.3FeCl.sub.2 +S
While this process is of interest for removing pyritic sulfur, a disadvantage of the process is that the liberated sulfur solids must then be separated from the coal solids. Processes involving froth flotation, vaporization and solvent extraction are proposed to separate the sulfur solids. All of these proposals, however, inherently represent a second discrete process step with its attendant problems and cost which must be employed to remove the sulfur from coal. In addition, this process is notably deficient in that it cannot remove organic sulfur from coal.
In another approach, U.S. Pat. No. 3,824,084 to Dillon, issued July 16, 1974, discloses a process involving grinding coal containing pyritic sulfur in the presence of water to form a slurry, and then heating the slurry under pressure in the pressence of oxygen. The patent discloses that under these conditions, the pyritic sulfur (for example, FeS.sub.2) can react to form ferrous sulfate and sulfuric acid which can further react to form ferric sulfate. The patent discloses that typical reaction equations for the process at the conditions specified are as follows: EQU FeS.sub.2 +H.sub.2 O+7/20.sub.2 .fwdarw.FeSO.sub.4 +H.sub.2 SO.sub.4 EQU 2FeSO.sub.4 +H.sub.2 SO.sub.4 +1/20.sub.2 .fwdarw.Fe.sub.2 (SO.sub.4).sub.3 +H.sub.2 O
These reaction equations indicate that in this particular process, the pyritic sulfur content continues to be associated with the iron as sulfate. Several factors detract from the desirability of this process. High temperatures and pressures are employed which can necessitate the use of expensive reaction vessels and processing plants of complex mechicanical design. Because high temperatures are employed, excessive amounts of energy can be expended in the process. In addition, the above oxidation process is not highly selective such that considerable amounts of coal itself can be oxidized. This is undesirable, of course, since the amount and/or heating value of the coal recovered from the process is decreased.
Heretofore, it was known that coal particles could be agglomerated with hydrocarbon oils. For example, U.S. Pat. No. 3,856,668 to Shubert, issued Dec. 24, 1974, and U.S. Pat. No. 3,665,066 to Capes et al, issued May 25, 1972, disclose processes for recovering coal fines by agglomerating the fine coal particles with oil. U.S. Pat. No. 3,268,071 to Puddington et al, issued Aug. 23, 1966, and U.S. Pat. No. 4,033,729, issued July 5, 1977, to Capes disclose processes involving agglomerating coal particles with oil in order to provide a separation of coal from ash. While these processes can provide some benefication of coal, improved ash and iron pyrite mineral matter removals would be desirable.
The above U.S. Pat. No. 4,033,729 to Capes et al relating to removing inorganic materials (ash) from coal significantly notes that iron pyrite mineral matter has proven difficult to remove because of its hydrophobic character. This disclosure confirms a long standing problem. The article "The Use of Oil in Cleaning Coal", Chemical and Metallurgical Engineering, Vol. 25, pages 182-188 (1921), discusses in detail cleaning coal by separating ash from coal in a process involving agitating coal-oil-water mixtures, but notes that iron pyrite is not readily removed in such a process.
In summary, while there is much prior art relating to processes for removing sulfur and ash from coal, there still exists a present need for a simple, efficient process for removing sulfur and ash from coal.